The present invention relates to a method of forming single crystal silicon-germanium alloy on a sapphire substrate. More specifically, but without limitation thereto, the present invention relates to the fabrication of single crystal silicon germanium alloy on a sapphire substrate for making integrated circuitry and electronic devices having improved performance characteristics.
The operating characteristics of CMOS and bipolar silicon devices may be enhanced by the implementation of silicon-germanium alloy structures. By careful control of the bandgap and engineered strain into the alloy heterostructure, significant performance enhancements may be realized. Strain is introduced into the structure when the heterogeneous alloy initially assumes the lattice structure spacing of the underlying surface material. As the thickness of the alloy film is increased, the alloy lattice structure relaxes, creating a network of misfit dislocations that may adversely affect the performance characteristics.
Devices that rely on hole conductance and confinement benefit from strained silicon-germanium alloys deposited on a relaxed silicon film, but n-channel devices are typically optimized using tensile strained silicon deposited on relaxed silicon-germanium alloys. Bulk silicon is of course readily available, but not silicon-germanium alloy substrates. Also, it would be highly desirable to have both relaxed silicon and silicon-germanium alloy on the same substrate to realize optimum performance from both n-type and p-type devices.
To obtain relaxed substrates of silicon and silicon-germanium alloy on a bulk silicon substrate requires elaborate and complicated film deposition sequences, including the deposition of thick graded buffer layers to achieve relaxed, defect-free silicon-germanium alloy on the bulk silicon substrate. The resulting topographic variation in circuit structure does not allow for thin film devices with their advantageous reduction in short channel effects, further contributing to the complexity of this process.
Silicon having a lattice orientation of 100! may be epitaxially deposited on the r-planes of sapphire designated as {1102} free of strain induced from the lattice mismatch, although approximately 0.3 to 0.4% compressive strain is introduced as a result of the high deposition temperature and the mismatch between the thermal coefficients of expansion of silicon and sapphire, respectively. The deposited films have a large number of stacking faults and twinning defects that may be eliminated by a sequence of silicon ion implantation and annealing steps. Some threading dislocations may remain in the resulting film, although this should not significantly affect performance. In addition, careful high resolution TEM studies indicate that the resulting interface is incoherent. Although evidence of misfit dislocations has been observed, the misfits are at the silicon-sapphire interface and well away from the conducting channel.
Pure germanium films have been grown on sapphire on different lattice orientations of sapphire {1102} and germanium 111!. Pure germanium grows as 110! on sapphire {1102}. Silicon-germanium 100! alloy films may also be grown epitaxially directly on a sapphire surface, though currently this process is more difficult than for pure silicon deposition. The silicon-germanium alloy film is believed to have similar properties to the pure silicon film, with the same lack of lattice induced strain and misfit dislocations. After the film reaches a critical thickness, the lattice structure assumes the spacing of the bulk crystal, and a relaxation of the interface with the sapphire crystal occurs so that approximately 10 percent lattice mismatch may be accommodated. However, during cooling from the deposition temperature and the accompanying reduction of silicon-germanium alloy atom mobility at the interface, the difference in thermal coefficients of expansion between the materials causes some strain in the film. The strain in the film is sufficient to increase the hole mobility and decrease the electron mobility beyond that observed in the bulk crystal.
In sufficiently thin silicon films grown directly on the surface of bulk or thick layer silicon-germanium alloy typically comprising 30 percent or more germanium, lattice mismatch occurs at the surface interface, forcing the silicon film to be in tensile stress. This differs from silicon and silicon-germanium alloy grown on sapphire in that there is no slip plane observed to relieve the stress at the interface. The tensile stress causes increased electron mobility and decreased hole mobility in the silicon film. In n-channel FET devices, the electron mobility is more important.
Epitaxial silicon germanium grown on a sapphire substrate at reduced temperatures of about 650 degrees C. was reported by Hiroyuki Wado et al in the article titled "Epitaxial growth of SiGe on Al.sub.2 O.sub.3 using Si.sub.2 H.sub.6 gas and Ge solid source molecular beam epitaxy" published in the Journal of Crystal Growth 169 (1996) pp. 457-462. A disadvantage of this method is that molecular beam epitaxy is not readily applicable to production processes.
A continuing need therefore exits for a method of fabricating a high quality silicon-germanium alloy on a sapphire substrate for optimizing both n-FET and p-FET devices fabricated on a common insulating substrate.